This invention relates to fault detection devices for detecting failures to acceleration sensors (G-sensors) and the starting means (squibs) of automotive passenger protection devices which detect impulses at the collision of automobiles and protect drivers and passengers by activating a passenger protection means such as an air bag and seat belt tensioner. More particularly, this invention relates to fault detection devices which is capable of individually detecting the failures of a plurality of squibs and acceleration sensors of the passenger protection devices.
Generally, the passenger protection devices such as automotive air bag system are activated when a strong impulse in excess of a predetermined magnitude is detected and the switch of an acceleration sensor is thus closed, thereby supplying a current above a predetermined level to a starting means, known as a squib, consisting of a kind of a heater (resistor). The passenger protection means is thus activated by the starting means. Then, an air bag is expanded instantaneously at the time of collision. Thus, a starter heater of a relatively small resistance value is utilized as the squib. Since failures of the circuit consisting of the acceleration sensor and the squib presents a grave danger for human lives, the circuit is continually monitored whether or not there is an abnormality with respect to the circuit voltage or the resistance value.
FIG. 10 is a circuit diagram showing a conventional fault detection device for a passenger protection device such as an automotive air bag system. This device is disclosed, for example, in Japanese Patent Publication (Kokoku) No. 61-57219.
In FIG. 10, a battery or DC power source 1 is mounted on an automobile. An ignition switch 2, which is coupled to the DC power source 1, is mechanically interlocked with the engine starter key of the automobile. A G-sensor or an acceleration sensor 3, coupled to the DC power source 1 via the ignition switch 2, consists of a resistor 3a and a normally open contact 3b. The normally open contact 3b is coupled in parallel with the resistor 3a and is closed upon detection of a collision. Squibs 4a and 4b for starting the protection operation (such as an air bag expansion operation) are coupled to the acceleration sensor 3 via a measurement point A. The squibs 4a and 4b are coupled in parallel circuit relationship. Another G-sensor or an acceleration sensor 5 is coupled at one terminal thereof to the squibs 4a and 4b via a measurement point B. The other terminal of the acceleration sensor 5 is grounded. The acceleration sensor 5 consists of a parallel circuit consisting of a resistor 5a and a normally open contact 5b.
A fault detection circuit 6 detects an occurrence of failure of the squibs 4a and 4b. The fault detection circuit 6 connects the acceleration sensors 3 and 5 and squibs 4a and 4b across the ignition switch 2 and the negative terminal of the DC power source 1. The fault detection circuit 6 consists of: a DC differential amplifier 7 coupled across the squibs 4a and 4b; and a judgement circuit 8 coupled to the output terminal of the differential amplifier 7, wherein the judgement circuit 8 is also connected across the ignition switch 2 and the negative terminal of the DC power source 1.
The differential amplifier 7 consists of: resistors 71 through 74 for adjusting the gain, and an operational amplifier 75. The resistor 71 is coupled across the measurement point A and the non-inverting input terminal of the operational amplifier 75. The resistor 72 is inserted between the ground and the non-inverting input terminal of the operational amplifier 75. The resistor 73 is inserted between the measurement point B and the inverting input terminal of the operational amplifier 75. The resistor 74 is inserted between the output terminal and the inverting input terminal of the operational amplifier 75.
An alarm lamp 9 coupled to an output terminal of the judgement circuit 8 is energized when the squibs 4a and 4b are in failure.
The operation of the fault detection device for a passenger protection device of FIG. 10 is as follows. When the ignition switch 2 is closed upon starting the automobile, the serially connected acceleration sensors 3 and 5 and the squibs 4a and 4b are supplied from the DC power source 1.
Usually, the normally open contacts 3b and 5b of the respective acceleration sensors are open, and current flows through the resistors 3a and 5a. It is necessary to limit the magnitude of the current such that no appreciable heat is generated by the squibs 4a and 4b. Thus, the resistance Rc of the resistors 3a and 5a are set above several hundred ohms. The composite effective resistance Rs of the squibs 4a and 4b is about the half of the individual resistances which are several ohms, respectively.
Across the two terminals of the squibs 4a and 4b is developed a voltage which is equal to the DC power source voltage V.sub.1 times the ratio of the composite effective resistance Rs of the squibs 4a and 4b and the composite effective resistance Rc of the resistors 3a and 5a. This voltage across the squibs is as small as about 10 mV. Thus, the differential amplifier 7 amplifies the voltage across the squibs 4a and 4b up to an easily measurable level and supplies the amplified output to the judgement circuit 8.
The judgement circuit 8 turns off the alarm lamp 9 when the resistances of the squibs 4a and 4b are normal, and turns the alarm lamp 9 on when an abnormality is detected upon failure, notifies the driver of an occurrence of abnormality.
On the other hand, in the case where the automobile is involved in a collisiion accident while the acceleration sensors 3 and 5 and the squibs 4a and 4b are functioning normally, the normally open contacts 3b and 5b are closed and the resistors 3a and 5a are thereby short-circuited. Thus, a large current is supplied to the squibs 4a and 4b. The heat thus generated by the squibs 4a and 4b starts the air bag expansion to protect the driver of the automobile. Under this circumstance, however, the failures of the acceleration sensors 3 and 5 cannot be detected.
FIG. 11 is a circuit diagram showing a conventional fault detection device for detecting an occurrence of failure of an acceleration sensor of a passenger protection device. The circuit is described, for example, in Japanese Patent Publication (Kokoku) No. 64-2537. The parts 1 through 5 and 8 and 9 are similar to those of FIG. 10 described above. It is noted however, that a single squib 4 is inserted between the acceleration sensors 3 and 5. The terminal voltages V.sub.A and V.sub.B of the respective acceleration sensors 3 and 5 coupled to the respective terminals of the squib 4 are directly applied to the judgement circuit 8.
A resistor 13 is inserted between the measurement point A of the acceleration sensor 3 and the squib 4 and the ground or the negative terminal of the DC power source 1. A resistor 14 is inserted between the measurement point B of the squib 4 and the acceleration sensor 5 and the positive terminal of the DC power source 1.
In the case of the circuit of FIG. 11, provided that the resistances of the two resistors 13 and 14 are equal to each other, the voltage at the measurement points A and B are half the DC power source voltage. If, however, one of the acceleration sensors 3 and 5 becomes abnormal and the resistance thereof changes, the voltages V.sub.A and V.sub.B at the measurement points A and B changes accordingly. Thus, upon detection of an abnormality, the judgement circuit 8 energizes the alarm lamp 9 immediately.
The above conventional fault detection devices, however, have the following disadvantages. Namely, since, in the case of the device of FIG. 10, the voltages at the two terminals A and B of a plurality of parallel-connected squibs 4a and 4b are monitored by the fault detection device, failures of individual squibs cannto be distinguished. Further, a small variation of the low voltage developed across the squibs caused by failures must be detected. Thus the gain of the differential amplifier 7 must be set at a large value, as a result, the fault detection device is easily affected by noises. Still further, since the reference level of the output of the differential amplifier 7 is the ground level, the amplified output is not determined when the squibs are operating normally, the errors such as offsets being included in the output.
In the case of the fault detection device of FIG. 11, the abnormality of the acceleration sensors are detected on the basis of the respective terminal voltages of the acceleration sensors 3 and 5. It is not possible, however, to monitor the resistances individually and to distinguish the occurrences of failures of a plurality of acceleration sensors.